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An object belonging to a class is a of if it does not belong to any subclass of . An object is a of a class if it is a direct member of or is a direct member of some subclass of . One of the advantages of object-oriented data models [BM93] compared to other data models is that they support a direct representation of real-world domains. These models directly represent the structure and behavior of real-world entities. Most object-oriented data models are based on the notion of class; objects with the same structure and behavior are grouped into classes, which are in turn organized in inheritance hierarchies. In most models, real-world entities are represented as instances of the most speci c (in the inheritance hierarchy) class in which they can be classi ed. Thus, real-world entities are partitioned into sets of disjoint classes. However, when modeling application domains, it is very frequent to come across objects which are direct members of multiple classes. In a model with multiple inheritance this situation can be handled by de ning a subclass of all the involved classes; this approach, however, leads to a large